Good traction on a flat, dry surface can be provided by a flat outsole made of a rubber or other elastomeric material. However, an outsole of this configuration typically provides poor traction on wet surfaces. To improve traction on wet surfaces, it has been known for many years to provide a pattern of wave-like, e.g., sinusoidal or zigzag, incisions (referred to as "siping") in the bottom surface of the outsole. Siping provides sharp edges when the sole is flexed, which tend to cut through the water and increase grip. It has also been known to provide lines of spaced ridges upon the bottom surface of an outsole, the ridges having sharp edges for gripping the walking surface to improve traction, e.g. upon wet surfaces.